1. Field of the Invention
The present invention concerns a method for generating magnetic resonance exposures (i.e. acquiring magnetic resonance data) of an examination subject, of the type wherein a dielectric element with high dielectric constant is positioned on the examination subject to locally influence the B1 field distribution. Moreover, the invention concerns a corresponding dielectric element for positioning on an examination subject for locally influencing the B1 field distribution during magnetic resonance data acquisition and a corresponding magnetic resonance system for implementation of the method.
2. Description of the Prior Art
Magnetic resonance tomography has become a widespread technique for the acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or a body part of the patient to be examined must initially be exposed to an optimally homogenous static basic magnetic field (usually designated as a B0 field) which is generated by a basic field magnet of the magnetic resonance scanner. During the acquisition of the magnetic resonance data, rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatially coding the data. With a radio-frequency antenna, RF pulses of a defined field strength are radiated into the examination volume of the scanner in which the examination subject is located. The magnetic flux density of these RF pulses is typically designated with B1. The pulsed radio-frequency field therefore is generally called a B1 field for short. By means of these RF pulses, the nuclear spins of the atoms in the examination subject are excited such that they are moved out of state of equilibrium spins, which are oriented parallel to the basic magnetic field B0, by what is known as an “excitation flip angle (also called “flip angle” for short in the following). The nuclear spins then precess in the direction of the basic magnetic field B0. The magnetic resonance signals thereby generated are acquired by radio-frequency reception antennas. The reception antennas can be either the same antennas with which the RF pulses were radiated, or separate antennas. The magnetic resonance images of the examination subject are ultimately created based on the received magnetic resonance signals. Every image point in the magnetic resonance image is associated with a small body volume, known as a “voxel”, and every brightness or intensity value of the image points is linked with the signal amplitude of the magnetic resonance signal received from this voxel. The connection between a radiated excitation RF pulse with the field strength B1 and the flip angle α achieved by this pulse is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic ratio, which can be considered as a fixed material constant for most magnetic resonance examinations, and τ is the effective duration of the radio-frequency pulse. The flip angle achieved by an emitted RF pulse, and thus the strength of the magnetic resonance signal, consequently also depends on (aside from the duration of the RF pulse) the strength of the radiated B1 field. Spatial fluctuations in the field strength of the excited B1 field therefore lead to unwanted variations in the received magnetic resonance signal that can adulterate the measurement result.
In the presence of a high magnetic field strength—that unavoidably exists in a magnetic resonance tomography scanner due to the necessary magnetic basic field B0—the RF pulses disadvantageously exhibit an inhomogeneous penetration behavior in conductive and dielectric media such as, for example, tissue. The B1 field thus can significantly vary within the measurement volume. In particular, in examinations known as ultra-intense field magnetic resonance examinations, in which modern magnetic resonance systems are used with a basic magnetic field of three Tesla or more, particular measures must be taken in order to achieve an optimally homogenous distribution of the transmitted RF field of the radio-frequency antenna in the entire volume.
A simple but more effective approach to the solution of the problem is to modify the electrical environment of the examination subject in a suitable manner in order to compensate unwanted inhomogeneities. For this purpose, dielectric elements with defined dielectric constant and conductivity can be positioned in the examination volume, for example directly at the patient or on the patient. The material of these dielectric elements should exhibit an optimally high dielectric constant, preferably ∈≧50. The dielectric material thus produces a dielectric focusing. Conversely, the material of the dielectric element should not exhibit too high a conductivity, since due to the skin effect a conductivity that is too high leads to high eddy currents, in particular in the surface region of the dielectric element, producing a shielding effect that weakens (attenuates) the dielectric focusing effect. For example, using such dielectric elements the typical RF field minima that occur in magnetic resonance examinations of a patient in the chest and abdomen region can be compensated by placing appropriate dielectric elements, which compensate the minima by producing a local increase of the penetrating radio-frequency field, on the chest and abdomen of the patient.
Distilled water with a dielectric constant of ∈≈80 and a conductivity of approximately 10 μs/cm and filled into a plastic film pouch has been used as such a dielectric element. Unfortunately, the use of such “dielectric pillows” filled with water has the unwanted side effect that they are visible in the magnetic resonance exposures. In addition, due to fold over effects the dielectric element may not be imaged within the magnetic resonance exposure at the location at which it is actually positioned in real space. Thus, for example, due to foldover effects the pillow can be shown at the upper edge of an MR image instead of at the lower edge. This leads to the impression being created on the magnetic resonance exposures that the dielectric element is located inside the body of the patient, rather than on the body of the patient. It is in principle possible to acquire an image using an oversampling method such that the dielectric element is at the correct position. In such a case, the dielectric element can be excised by image processing an image section can be selected which does not contain the dielectric element at all. These oversampling methods, however, are quite time-consuming and therefore prolong the measurement time. Moreover, independent of the oversampling methods, given movement (for example due to breathing of the patient) the MR signal from the pillow can also lead to interfering image artifacts within the region of the magnetic resonance exposure depicting the body.